The present invention relates to ultrasonic flow measurement, in particular a device and method for measuring a flow characteristic, e.g. flow velocity and/or throughput of a fluid in a conduit.
Ultrasonic flow measurement of fluids flowing in a conduit is generally known in the art. Basically it comprises transmitting ultrasonic sound waves between a transmitter and a receiver, that are spaced apart in axial direction of a conduit, in a downstream direction of the fluid flow and in the upstream direction thereof. The transit time of the ultrasonic waves is measured. From the difference in transit time of the downstream directed ultrasonic wave and the upstream directed wave an average flow velocity or throughput can be calculated based on a known geometry of the conduit. The fluid is a gas, vapour or liquid, such as natural gas.
In addition to the flow velocity, the transit time as measured is dependent from the flow profile of the fluid in the conduit. Swirl, cross flow, profile asymmetry and a velocity fluctuating in time are known distortions from an ideal flow profile, which frequently occur in practice, e.g. in complex piping structures, after bends and the like.
The ultimate reliability of the flow velocity as measured and calculated depends on many parameters, like the distance covered, the acoustic path configuration, transmitted ultrasonic wave type and the calculation method itself. Many acoustic path configurations are known in the art.
One of the known acoustic path configurations is a triangular path having midradius chords, wherein the ultrasonic wave transmitted by the transmitter reflects twice at the inner conduit wall prior to being received by the receiver. E.g. U.S. Pat. No. 5,546,812 has disclosed a method and device for determining characteristics of the flow of a medium in a channel, comprising a transducer arrangement defining two triangular paths offset to each other for swirl determination and three single reflection axial paths also offset to each other for (a)symmetry determination. In commercially available flow meters according to this patent, the first triangular path has a clockwise orientation and the second triangular path has a counter clockwise orientation.
Despite the well recognized excellent performance of these known devices, some tests have shown that the accuracy of the measured characteristics, in particular flow velocity and diagnostics such as swirl angle and asymmetry derived from the swirl paths or combination thereof, leaves something to be desired in some situations. This is particularly true if there is an asymmetry in a certain orientation or if the flow profile changes along the axial distance of the triangular acoustic path, thereby affecting the flow measurement accuracy. Using these known devices it is not possible to fully compensate or eliminate such an effect to an acceptable level. Moreover, in general demands on reducing uncertainty and on improving accuracy of flow meters still increase. Therefore there is a continuing need for improving the accuracy of ultrasonic flow meters and reducing the uncertainty thereof.